1. Field of the Invention
The present invention relates generally to nuclear reactors, and more particularly, to nuclear reactors having fuel assemblies that employ support grids.
2. Description of the Related Art
In most water cooled nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. In pressurized water nuclear reactors, these fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of support grids spaced axially along the fuel assembly length and attached to a plurality of elongated thimble tubes. The thimble tubes typically receive control rods or instrumentation therein. Top and bottom nozzles are on opposite ends of the fuel assembly and are secured to the ends of the thimble tubes that extend slightly above and below the ends of the fuel rods.
The grids, as is known in the relevant art, are used to precisely maintain the spacing and support between the fuel rods in the reactor core, provide lateral support for the fuel rods and induce mixing of the coolant. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of roughly square cells which individually accept the fuel rods therein. Depending upon the configuration of the thimble tubes, the thimble tubes can either be received in the cells that are sized the same as those that receive fuel rods therein, or in relatively larger thimble cells defined in the interleaved straps. The interleaved straps provide attachment points to the thimble tubes, thus enabling their positioning at spaced locations along the length of the fuel assembly.
During the fuel rod loading process, a diligent effort is made to help assure that physical contact and dynamic friction between the rod and grid is reduced or eliminated. Nuclear fuel fabrication has demonstrated gall-ball formation at the interface between the fuel rod and the rod support features of the grid. The presence of large gal-balls at these interfaces, given the appropriate flow conditions and cycle lengths, could contribute to the formation of gaps between the fuel rod and the fuel rod support system. For example, the formation of large gaps could result in an increased rate of grid-to-rod fretting and damage of the fuel rod. In particular, in at least one nuclear reactor, it was noted that the presence of gal-balls resulted in the deepening of wear scars on the fuel rods.
Accordingly, a means of reducing or minimizing the size and quantity of gal-balls during the fuel rod loading and assembly process is desired to: (i) reduce the risk of grid-to-rod fretting resulting from the presence of gal-balls within the grid space of a nuclear fuel assembly and/or (ii) reduce the amount of debris in the reactor coolant to potentially reduce fuel assembly damage and other reactor internal damage created by substantial flow forces and circulation of debris.